There are no physical boundaries in space. The solar system consists of eight planets orbiting around one star: the Sun. Neptune, the farthest planet from the Sun, orbits approximately 30astronomical units from the Sun. An astronomical unit is a unit of length used by astronomers. One astronomical unit equals the distance from Earth to the Sun: 93 million miles (149 million km). Some of the comets associated with the solar system travel on orbits that take them much farther from the Sun than Neptune.

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